Radiographic elements can serve a variety of imaging applications, but are most extensively employed in medical diagnostic imaging. The radiographic elements produce viewable silver images upon imagewise exposure followed by rapid access processing.
Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. The discovery led to medical diagnostic imaging. In 1913 the Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation. Shortly thereafter it was discovered that the films could be more efficiently employed in combination with one or two intensifying screens. An intensifying screen is relied upon to capture an image pattern of X-radiation and emit light that exposes the radiographic element. Elements that rely entirely on X-radiation absorption for image capture are referred to as direct radiographic elements, while those that rely on intensifying screen light emission, are referred to as indirect radiographic elements. Silver halide radiographic elements, particularly indirect radiographic elements, account for the overwhelming majority of medical diagnostic images.
In recent years a number of alternative approaches to medical diagnostic imaging, particularly image acquisition, have become prominent. Medical diagnostic devices such as storage phosphor screens, CAT scanners, magnetic resonance imagers (MRI), and ultrasound imagers allow information to be obtained and stored in digital form. Although digitally stored images can be viewed and manipulated on a cathode ray tube (CRT) monitor, a hard copy of the image is almost always needed.
The most common approach for creating a hard copy of a digitally stored image is to expose a radiation-sensitive silver halide film through a series of laterally offset exposures using a laser, a light emitting diode (LED) or a light bar (a linear series of independently addressable LED's). The image is recreated as a series of laterally offset pixels. Initially the radiation-sensitive silver halide films were essentially the same films used for radiographic imaging, except that finer silver halide grains were substituted to minimize noise (granularity). The advantages of using modified radiographic films to provide a hard copy of the digitally stored image are that medical imaging centers are already equipped for rapid access processing of radiographic films and are familiar with their image characteristics.
Rapid access processing can be illustrated by reference to the Kodak X-OMAT M6A-N.TM. rapid access processor, which employs the following (reference) processing cycle:
development 24 seconds at 35.degree. C. PA1 fixing 20 seconds at 35.degree. C. PA1 washing 20 seconds at 35.degree. C. PA1 drying 20 seconds at 65.degree. C. PA1 hydroquinone 30 g PA1 1-phenyl-3-pyrazolidone 1.5 g PA1 KOH 21 g PA1 NaHCO.sub.3 7.5 g PA1 K.sub.2 SO.sub.3 44.2 g PA1 Na.sub.2 S.sub.2 O.sub.3 12.6 g PA1 NaBr 35.0 g PA1 5-methylbenzotriazole 0.06 g PA1 glutaraldehyde 4.9 g PA1 water to 1 liter at a pH 10.0. PA1 Na.sub.2 S.sub.2 O.sub.3 in water at 60% of total weight PA1 in water 260.0 g PA1 NaHSO.sub.3 180.0 g PA1 boric acid 25.0 g PA1 acetic acid 10.0 g PA1 water to 1 liter at a pH of 3.9-4.5. Numerous variations of the reference processing cycle (including, shorter processing times and varied developer and fixer compositions) are known. PA1 the reference processing cycle consisting of PA1 hydroquinone 30 g PA1 1-phenyl-3-pyrazolidone 1.5 g PA1 KOH 21 g PA1 NaHCO.sub.3 7.5 g PA1 K.sub.2 SO.sub.3 44.2 g PA1 Na.sub.2 S.sub.2 O.sub.3 12.6 g PA1 NaBr 35.0 g PA1 5-methylbenzotriazole 0.06 g PA1 glutaraldehyde 4.9 g PA1 water to 1 liter at a pH 10.0, PA1 Na.sub.2 S.sub.2 O.sub.3 in water at 60% of total weight 260.0 g PA1 NaHSO.sub.3 180.0 g PA1 boric acid 25.0 g PA1 acetic acid 10.0 g PA1 water to 1 liter at a pH of 3.9-4.5. PA1 Maskasky U.S. Pat. No. 4,400,463; PA1 Maskasky U.S. Pat. No. 4,713,323; PA1 Takada et al U.S. Pat. No. 4,783,398; PA1 Nishikawa et al U.S. Pat. No. 4,952,491; PA1 Ishiguro et al U.S. Pat. No. 4,983,508; PA1 Tufano et al U.S. Pat. No. 4,804,621; PA1 Maskasky U.S. Pat. No. 5,061,617; PA1 Maskasky U.S. Pat. No. 5,178,997; PA1 Maskasky and Chang U.S. Pat. No. 5,178,998; PA1 Maskasky U.S. Pat. No. 5,183,732; PA1 Maskasky U.S. Pat. No. 5,185,239; PA1 Maskasky U.S. Pat. No. 5,217,858; PA1 Chang et al U.S. Pat. No. 5,252,452; PA1 Maskasky U.S. Pat. No. 5,264,337; PA1 Maskasky U.S. Pat. No. 5,272,052; PA1 Maskasky U.S. Pat. No. 5,275,930; PA1 Maskasky U.S. Pat. No. 5,292,632; PA1 Maskasky U.S. Pat. No. 5,298,387; PA1 Maskasky U.S. Pat. No. 5,298,388; and PA1 House et al U.S. Pat. No. 5,320,938. PA1 Abbott et al U.S. Pat. No. 4,425,425; PA1 Abbott et al U.S. Pat. No. 4,425,426; PA1 Kofron et al U.S. Pat. No. 4,439,520; PA1 Maskasky U.S. Pat. No. 4,713,320; PA1 Nottorf U.S. Pat. No. 4,722,886; PA1 Saito et al U.S. Pat. No. 4,797,354; PA1 Ellis U.S. Pat. No. 4,801,522; PA1 Ikeda et al U.S. Pat. No. 4,806,461; PA1 Ohashi et al U.S. Pat. No. 4,835,095; PA1 Makino et al U.S. Pat. No. 4,835,322; PA1 Daubendiek et al U.S. Pat. No. 4,914,014; PA1 Aida et al U.S. Pat. No. 4,962,015; PA1 Tsaur et al U.S. Pat. No. 5,147,771; PA1 Tsaur et al U.S. Pat. No. 5,147,772; PA1 Tsaur et al U.S. Pat. No. 5,147,773; PA1 Tsaur et al U.S. Pat. No. 5,171,659; PA1 Black et al U.S. Pat. No. 5,219,720; PA1 Antoniades et al U.S. Pat. No. 5,250,403; PA1 Dickerson et al U.S. Pat. No. 5,252,443; PA1 Tsaur et al U.S. Pat. No. 5,272,048; PA1 Delton U.S. Pat. No. 5,310,644; PA1 Chaffee et al U.S. Pat. No. 5,358,840; PA1 Delton U.S. Pat. No. 5,372,927; PA1 Delton U.S. Pat. No. 5,460,934; PA1 Daubendiek et al U.S. Pat. No. 5,494,789; PA1 Olm et al U.S. Pat. No. 5,503,970; PA1 Daubendiek et al U.S. Pat. No. 5,503,971; PA1 Daubendiek et al U.S. Pat. No. 5,573,902; PA1 Daubendiek et al U.S. Pat. No. 5,576,168; PA1 Olm et al U.S. Pat. No. 5,576,171; PA1 Deaton et al U.S. Pat. No. 5,582,965; and PA1 Wilson et al U.S. Pat. No. 5,614,358.
with up to 6 seconds being taken up in film transport between processing steps.
A typical developer employed in this processor exhibits the following composition:
A typical fixer employed in this processor exhibits the following composition:
Rapid access processors are typically activated when an imagewise exposed element is introduced for processing. Silver halide grains in the element interrupt an infrared sensor beam in the wavelength range of from 850 to 1100 nm, typically generated by a photodiode. The silver halide grains reduce density of infrared radiation reaching a photosensor, telling the processor that an element has been introduced for processing and starting the rapid access processing cycle. Once silver halide grains have been developed, developed silver provides the optical density necessary to interact with the infrared sensors. When the processed element emerges from the processor, an infrared sensor placed near the exit of the processor receives an uninterrupted infrared beam and shuts down the processor until another element is introduced requiring processing.
Highly advantageous silver halide emulsions for forming silver images in radiographic elements are tabular grain emulsions. Among their many advantages, tabular grain emulsions exhibit high levels of covering power (the ratio of maximum density divided by silver coating coverage), as illustrated by Dickerson U.S. Pat. No. 4,414,304. The covering power of tabular grain emulsions increases as the mean thickness of the tabular grains decreases. The high covering power of very thin (0.10 .mu.m or less) tabular grain emulsions allows them to be coated in radiographic elements at silver coverages of less than 30 mg/dm.sup.2. The low silver coating coverages in turn allow radiographic element constructions that can be processed in less than 45 seconds and even less than 30 seconds.
While lower silver coating coverages are in themselves advantageous in saving materials and facilitating rapid access processing, the low silver coverages have presented a problem in using commercially available rapid access processors, since they lack sufficient infrared density to be detected by the sensor beams used to sense the presence of radiographic film in rapid access processors.
Harada et al U.S. Pat. No. 5,260,178 has noted that with low silver coating coverages in radiographic elements, it is impossible for sensors that rely on the scattering of near infrared sensor beams by silver halide grains to sense the presence of the film in the processor. The solution proposed is to incorporate an infrared absorbing dye. Instead of reducing specular density by scattering near infrared radiation, the dye simply absorbs the near infrared radiation of the sensor beam. During processing the dye is deaggregated to shift its absorption peak. In the later stages of processing the density of developed silver is relied upon for interrupting sensor beams, which is the conventional practice.
The difficulty with the Harada et al solution to the problem of insufficient silver halide grain coating coverages to activate infrared sensors is that it relies on the addition of a complex organic material--specifically a tricarbocyanine dye that must have, in addition to the required chromophore for near infrared absorption, a steric structure suitable for aggregation and solubilizing substituents to facilitate deaggregation. The dyes of Harada et al also present the problem of fogging the radiation-sensitive silver halide grains when coated in close proximity, such as in a layer contiguous to a radiation-sensitive emulsion layer. Simply stated, the burden of the "cure" that Harada proposes is sufficiently burdensome as to entirely offset the advantage of reduced silver coating coverages, arrived at by years of effort by those responsible for improving films for producing silver images in response to rapid access processing. Thus, Harada's film structure modification is not a problem solution that has practical appeal.